Bone fixation device

ABSTRACT

A bone fixation device comprising: first and second screw receiving members, each member having an aperture extending therethrough for receiving a screw; and a plastically deformable connecting arm extending between the screw receiving members along a long axis defined by the first and second screw receiving members; the arm being adapted to be more pliable than the screw receiving members such that (a) on twisting the device about its long axis the arm twists before the screw receiving members deform; and (b) on bending the device along its long axis the arm bends before the screw receiving members deform.

The present invention relates to a bone fixation device. Moreparticularly, but not exclusively, the present invention relates to abone fixation device comprising first and second screw receiving membersconnected together by a relatively pliable connecting arm adapted tobend before the screw receiving members.

Bone fixation plates are known. Such plates typically comprise aplastically deformable plate such as a metal plate. A plurality ofapertures extend through the plate. In use the plate is positionedagainst a bone and screws passed through the apertures to fix the plateto the bone.

In order to ensure a close fit between the plate and the bone it isoften necessary to deform the plate slightly. Deforming the platedistorts the apertures, this reduces the accuracy of the fit betweenplate and screws and may allow the plate to move relative to the boneduring use.

A further bone fixation device is disclosed in PCT/US2005/020157. Such adevice comprises a number substantially spherical screw receivingmembers having screws extending therethrough. Extending from each screwreceiving member is a connecting member. A connecting arm, rather thanextending along a long axis between the screw receiving members, extendsto the side of the long axis. The screw receiving members are connectedto the connecting arm by the connection members. Such a construction hasa number of drawbacks. It is complex having a number of parts which mustbe assembled in a multistep procedure. In addition, one cannot simplytwist the device about the long axis as is often required.

The device according to the invention seeks to overcome these problems.

Accordingly, the present invention provides a bone fixation devicecomprising

-   -   first and second screw receiving members, each member having an        aperture extending therethrough for receiving a screw; and    -   a plastically deformable connecting arm extending between the        screw receiving members along a long axis defined by the first        and second screw receiving members;    -   the arm being adapted to be more pliable than the screw        receiving members such that (a) on twisting the device about its        long axis the arm twists before the screw receiving members        deform; and (b) on bending the device along its long axis the        arm bends before the screw receiving members deform.

The device according to the invention can be attached by a simplifiedprocedure whereby it is deformed to the correct shape without deformingthe apertures, placed against the bone and then screwed in place. Inaddition, it can be both bent along its long axis and twisted about itslong axis to correctly align the device with the bone to the patient.

The device according to the invention is deformed to fit a bone theconnecting arm(s) will deform before the screw receiving members. Thisallows the device to be deformed to the correct shape without theapertures becoming distorted.

Preferably, the device comprises a plurality of screw receiving membersconnected together in a line by connecting arms.

Preferably, at least one connecting arm is cylindrical in cross section.

At least one screw receiving member can be substantially spherical.

Preferably, a portion of the spherical member is flattened.

The flattened part can be centered about a mouth of the aperture and isin a plane normal to the axis of the aperture.

Preferably, a part of at least one aperture is threaded for receiving ascrew.

Preferably, the aperture is cylindrical.

The cross section of the cylindrical aperture can be constant along itslength.

Preferably, the device further comprises at least one screw adapted forthreaded engagement with the threaded portion of the aperture, the screwhaving a tapered head adapted to be drawn into the aperture and to pressfit with the walls of the aperture on threading the screw intoengagement with the threaded portions of the aperture.

Alternatively, the cylindrical aperture has at least one step and crosssection along its length to define a lip for engagement with a taperedhead of a screw.

In a further aspect of the invention there is provided a method ofconnecting a bone fixation device to a bone comprising the steps of

-   -   (a) providing a bone fixation device comprising        -   first and second screw receiving members, each member having            an aperture extending therethrough for receiving a screw;            and        -   a plastically deformable connecting arm extending between            the screw receiving members along a long axis defined by the            first and second screw receiving members;        -   the arm being adapted to be more pliable than the screw            receiving members such (a) on twisting the device about its            long axis the arm twists before the screw receiving members            deform; and (b) on bending the device along its long axis            the arm bends before the screw receiving members.    -   (b) deforming the bone fixation device to the profile of the        bone; and,    -   (c) screwing the bone fixation device to the bone by means of        screws inserted through the apertures on the screw receiving        members.

The present invention will now be described by way of example only andnot in any limitative sense with reference to the accompanying drawingsin which

FIG. 1 shows a known fixation plate before and after deformation;

FIG. 2 shows a further known bone fixation device in perspective view;

FIG. 3 shows a bone fixation device according to the invention inperspective view;

FIG. 4 shows the embodiment of FIG. 3 in combination with a screw;

FIG. 5 shows a further embodiment of a bone fixation device according tothe invention in cross sectional view; and

FIG. 6 shows the embodiment of FIG. 5 after twisting.

Shown in FIG. 1 is a known bone plate 1. The bone plate 1 comprises ametal plate 2 having a plurality of apertures 3. In use the bone plate 1is placed against a bone (not shown). Bone fixation screws (not shown)are passed through the apertures 3 and then screwed into the bone. Thescrew heads typically engage with the plate 1 firmly fixing the plate 1in position and preventing it from being displaced with respect to thebone.

A problem can arise however if the bone plate 1 needs to be deformedbefore it can be fixed to the bone. Deformation of the plate 1 deformsthe apertures 3 in the plate 1 as shown. This prevents the screw headsfrom accurately engaging with the apertures 3 which may result in theplate 1 being free to wobble slightly with respect to the bone. This canreduce the effectiveness of the bone plate 1 as a support for the bone.

In addition, it can be difficult to remove such a known bone plate 1from the bone when it is no longer required. The screw heads tend tocold weld to the bone plate 1 over time making the screws difficult toremove. It is often necessary to cut the bone plate 1 free which canresult in damage to the bone.

Such a known bone plate 1 is also limited as to how it can be deformed.Whilst the plate 1 can be bent as shown in FIG. 1 it is not a simplematter to twist it such that the apertures 3 lie in different planes.

Shown in FIG. 2 is a further known bone fixation device 4. This device 4comprises a plurality of substantially spherical screw receiving members5. Extending from each screw receiving member 5 is a connecting member6. A connecting arm 7 extends through the connecting members 6connecting the screw receiving members 5 together.

In use the screw receiving members 5 are arranged in the correctposition and screwed to the bone. The connecting arm 7 is then passedthrough the connecting members 6 to complete the device 4.

Such a device 4 overcomes the problem of deformation of the screwreceiving members by separating the screw receiving members 5 from theconnecting arm 7 to be deformed. This separation however increases thecomplexity of the device 4 and the installation procedure. Theconnecting arm 7 needs to be deformed to a complex shape before it canbe threaded through the connecting members 6. The threading can bedifficult or even impossible in a confined space. Because of the gapbetween the screw receiving members 5 and the connecting arm 7 thedevice 4 is unsuitable for use where a longitudinal twist of the device4 is required.

Shown in FIG. 3 is a bone fixation device 8 according to the invention.The device comprises a plurality of screw receiving members 9. Apertures10 extend through each of the screw receiving members 9 for receivingscrews. A long axis 11 extends between each of the screw receivingmembers 9. Plastically deformable connecting arms 12 extend along thelong axis 11 between the screw receiving members 9 as shown. In thisembodiment the long axis 11 passes through the apertures 10 of the screwreceiving members 9.

In use the bone fixation device 8 is gripped and bent to the requiredshape. The arms 12 are more pliable than the screw receiving members 9and accordingly it is the arms 12 that bend when the force is applied,rather than the screw receiving members 9.

The apertures 10 therefore remain undistorted. In addition, in contrastto known bone plates 1 a torsional (twisting) force can be applied tothe device 8 rotating one or more of the screw receiving members 9 aboutthe long axis 11 of the device 8 if required. As the long axis 11 passesalong the length of the connecting arm 12 the connecting arm 12 twistsabout its length. The device 8 can therefore be twisted withoutsignificantly altering its dimensions. The device 8 can therefore beinserted into small apertures even after twisting.

In this embodiment of the invention each of the screw receiving members9 is substantially spherical with the apertures 10 extending through thecentres of the spheres 9. Each aperture 10 intersect the sphere atmouths 13 on opposite sides of the sphere 9. The sphere 9 comprises aslightly flattened portion 14 around one of the mouths. This reduces theprofile of the device 8. It also provides an extended contact areabetween the screw receiving members 9 and the bone (not shown).

The connecting arms 12 between the screw receiving members 9 arecylindrical. The interface 15 between the arms 12 and spherical screwreceiving members 9 is chamfered so that any bending or torsional forcesdo not concentrate at this interface 15.

The ends 16 of the device 8 are tapered as shown so that the device 8can be placed between bone and soft tissue without surgically exposingthe entire length of bone.

Each of the apertures 10 of this embodiment is cylindrical having aconstant area along its length. A portion of the aperture 10 isthreaded. The remainder of the aperture 10 is smooth walled.

After bending and/or twisting to the correct shape the device 8 ispositioned against the bone. Screws (not shown) are inserted into theapertures 10 through the smooth portions and into threaded engagementwith the threaded portions. On further rotation of the screws theypenetrate and grip the bone, fixing the device 8 to the bone. Asignificant advantage of the device 8 is that it can be bent/twisted tothe correct shape, positioned correctly and then screws inserted. Thisconsiderably simplifies the attachment procedure. As the screw receivingmembers 9 are aligned with the connection arms 12 along the long axis 11the device 8 can be twisted about its length without any significantchange in dimensions of the device 8. This is particularly useful wheninserting the device 8 into small apertures.

The device 8 is adapted to be used with a screw (not shown) having twoportions—a threaded portion for gripping the threaded portion of theaperture 10 and then the bone and a smooth portion extending from thethreaded portion . The smooth portion has an outer face which issubstantially cylindrical and of the same diameter as the threadedportion. The smooth portion is however slightly tapered with itsdiameter increasing in a direction away from the threaded portion. Atits end the diameter of the smooth portion is slightly larger than thediameter of the aperture 10. As the screw is turned and is drawn intothe aperture 10 the smooth portion of the screw abuts the smooth portionof the aperture 10 so producing a press fit.

An alternative embodiment of the invention is shown in FIG. 4. In thisembodiment the threaded portion of the aperture 10 is narrower indiameter than the smooth portion. The screw 17 has a narrow threadedportion 18 and a wider smooth tapered head portion 19 as shown. Thenarrow threaded portion 18 of the screw 17 engages with the narrowportion of the aperture 10 drawing the larger smooth head portion 19into abutment with the smooth portion of the aperture 10.

The embodiment of FIG. 4 can be used with an alternative design of screw(not shown). The alternative design comprises a threaded shaft having adomed head extending away from the shaft. As the screw is drawn into theaperture 10 the domed head abuts the lip formed by the change indiameter of the aperture 10.

In a further embodiment of the invention (not shown) both the narrow andwide portions of the aperture 10 are threaded. Similarly, the screwcomprises a narrow threaded shaft and a larger diameter threaded head.In use the threaded head engages with the larger portion of the aperture10.

In a further embodiment of the invention (not shown) a portion of theaperture 10 is slightly bevelled so that a standard bone screw head willpress fit into engagement with the bevels as the screw is drawn into theaperture 10.

Shown in FIG. 5 in cross sectional view is a further embodiment of adevice 8 according to the invention. The aperture 10 comprises a firstnarrow threaded section 20. Extending from this is a second smoothwalled section 21 of larger diameter. Extending from the second section21 is a third smooth walled section 22 of slightly larger diameter thenthe second section 21. The step change in diameter from the secondsection 21 to the third section 22 defines a lip 23. In use a screw 24is inserted into the aperture 10 with the threaded portion 25 of thescrew 24 in threaded engagement with the narrower portion 21 of theaperture 10. As the screw 24 is turned and drawn into the aperture 10the head 26 of the screw 24 abuts the lip 23 producing a tight fit.

Shown in FIG. 6 is the embodiment of FIG. 5 in perspective view. Thedevice 8 has been twisted along its length such that the apertures 10lie in different planes. As can be seen, the arms 12 have twisted beforethe screw receiving members 9 deform.

In a further embodiment of the invention the screw receiving members 9are substantially elliptical.

The device 8 according to the invention can be used with any tool whichgrips the screw receiving members 9. One preferred embodiment of such atool comprises jaws having cut out sections which match the sphericalcomponent of the screw receiving members 9. The tool also has cut outswhich match the interface 15 between the spherical component 9 andconnecting arm 12 so that the tool can apply a bending force throughoutthe length of the bone fixation device 8. In an alternative embodimentthe jaws have cut outs which match the flattened portions 14 of thescrew receiving members 9.

In use two of the tools are used to grip the screw receiving members 9.The device 8 is then bent and/or twisted to the desired shape and thenreleased.

1. A bone fixation device comprising: first and second screw receivingmembers, each member having an aperture extending therethrough forreceiving a screw; and a plastically deformable connecting arm extendingbetween the screw receiving members along a long axis defined by thefirst and second screw receiving members; the arm being adapted to bemore pliable than the screw receiving members such that (a) on twistingthe device about the long axis the arm twists before the screw receivingmembers deform; and (b) on bending the device along the long axis thearm bends before the screw receiving members deform.
 2. A bone fixationdevice as claimed in claim 1, comprising a plurality of screw receivingmembers connected together in a line by connecting arms.
 3. A bonefixation device as claimed in claim 1, wherein at least one connectingarm is cylindrical in cross section.
 4. A bone fixation device asclaimed in claim 1, wherein at least one screw receiving member issubstantially spherical.
 5. A bone fixation device as claimed in claim4, wherein the spherical screw receiving member includes a flattenedportion.
 6. A bone fixation device as claimed in claim 5, wherein theflattened portion is centered about a mouth of the aperture and is in aplane normal to the axis of the aperture.
 7. A bone fixation device asclaimed in claim 1, wherein at least one aperture includes a threadedportion for receiving a screw.
 8. A bone fixation device as claimed inclaim 7, wherein the aperture is cylindrical.
 9. A bone fixation deviceas claimed in claim 8, wherein the cross section of the cylindricalaperture is constant along a length of the aperture.
 10. A bone fixationdevice as claimed in claim 9, further comprising at least one screwadapted for threaded engagement with the threaded portion of theaperture, the screw having a tapered head adapted to be drawn into theaperture and to press fit with walls of the aperture on threading thescrew into engagement with the threaded portion of the apertures.
 11. Abone fixation device as claimed in claim 8, wherein the cylindricalaperture has at least one step in cross section along a length of theaperture to define a lip for engagement with a tapered head of a screw.12. A method of connecting a bone fixation device to a bone utilizingscrews, said method comprising the steps of: providing a bone having aprofile; providing a bone fixation device comprising: first and secondscrew receiving members, each member having an aperture extendingtherethrough, a plastically deformable connecting arm extending betweenthe screw receiving members; deforming the bone fixation device to theprofile of the bone; screwing the bone fixation device to the bone bymeans of screws inserted through the apertures on the screw receivingmembers.